Help with feeding fuel through docking ports

[Joe32320]

I'm having some problems with a ship I'm building in orbit around Kerbin, with docking ports not allowing fuel to move across tanks towards the engine.

The ship as you can see below has a central stack with two engine modules attached to the bottom tanks via two medium size docking ports.

However when the engines are lit, only fuel in the 2 engine modules is used, and the fuel in the central tanks is unused, and I'm at a loss as to why this should be happening, is there something I'm doing wrong?

I have tried rebuilding in the VAB and testing it on the launchpad and it seems to be ok there, though the difference there was that I was forced to use decouplers instead of 2 docking ports to connect the engine module because I can't find a way to connect a fuel tank to a docking port, and run a fuel line from the central tanks to the engine modules, so my suspicions are that the problem rests with the docking ports connecting the engine modules to the central stack.

[Captain Sierra]

What's going on here is the way fuel feeds. Fuel will feed outwards radially only to structural parts, not in. I'm not exactly sure how to correct the issue in this particular instance but I can suggest that you run fuel lines to each of teh docking ports and then back to the tank, thus ensuring the docking ports have two way flow capacity.

[enki]

run fuel lines to each of the docking ports and then back to the tank, thus ensuring the docking ports have two way flow capacity.

Fuel loops won't help things.

If OP isn't averse to adding new mods, TAC Fuel Balancer is a better way than manually shifting fuel to get it in the right tanks. OP can try right clicking on the docking ports to confirm that fuel cross feed is enabled and it should be able to flow into the engines. I made a testbed to explore the behavior of this design. Unless there is something wildly different with something assembled entirely in the SPH vs. something assembled from separate pieces the fuel was used as it should have been, emptying the fuel tank furthest from the engines first and then moving to the next.

[Rhomphaia]

Fuel loops won't help things.

If OP isn't averse to adding new mods, TAC Fuel Balancer is a better way than manually shifting fuel to get it in the right tanks. OP can try right clicking on the docking ports to confirm that fuel cross feed is enabled and it should be able to flow into the engines. I made a testbed to explore the behavior of this design. Unless there is something wildly different with something assembled entirely in the SPH vs. something assembled from separate pieces the fuel was used as it should have been, emptying the fuel tank furthest from the engines first and then moving to the next.

Actually there is something different in your design. Your docking ports are stack attached to the fuel tanks on the engine pods whereas the OPs docking ports are radial children of the fuel tank. Fuel can only flow radially from parent to child.

So in your design the tanks in the stack can feed the engine, but the forward radial tanks cannot since they can't feed fuel back into the main stack.

The OP's design should not need many fuel lines, on the engine modules running from one set of docking ports to the fuel tank should do it.

Assuming of course that docked ports behave the same as assembled ones.

[Kasuha]

I had similar setup, just with engines in the middle and fuel tanks on sides. I found that fuel feeds through docking port only when it is mounted to the tank's attachment point, not radially. Or something like that, I'm not sure. I ended up pumping fuel between tanks to feed the engine.

[Leyvin]

Fuel will only flow via the Docking Ports provided the part count is below 250 at Launch... why this is the case has confused the f**k out of me for weeks, as it was how I built my NASA-Style Shuttle - with one launch it worked, the after that it just stopped working for no reason (along with the body engines despite fuel lines being setup on it) causing me to completely redesign it.

Leads me to believe this is bugged behaviour and they should still be working correctly but Part Count will cause them to be ignored as if they're not there.